Every so often a supporting actor takes such a huge bite out of a movie that afterwards you can only remember the silhouette of the bite-mark, and nothing of the film that got bit. That's how Amy Ryan first caught our attention, when she showed up in minute 30 of Ben Affleck's otherwise only so-so Dennis Lehane adaptation Gone Baby Gone, and just owned that movie for the next seven minutes.

Playing the malevolent, abrasive junkie single mother of a missing kidnap victim, a slatternly, slack-jawed racist, Ryan adopted a drunkard's waxen pallor, honked up the full braying working-class Boston accent and, in those seven minutes, ran a gamut of emotions, from sullen resentment to inappropriate levity and a final descent into abject sobbing – a magnificent shipwreck of a performance. Best Supporting Actress Oscar nomination duly earned (though the award went in the end to Tilda Swinton for her icy lawyer in Michael Clayton).

Amy Ryan gets hired by people with really good taste in actors, people such as Lodge Kerrigan, the makers of The Wire and The Office, Sidney Lumet, Albert Brooks, Paul Greengrass and Clint Eastwood. And now by Philip Seymour Hoffman – an actor's actor's actor, so to speak – in his low-key, low-budget directorial debut, Jack Goes Boating. Hoffman and Ryan are a nervous, virginal couple who tentatively build a relationship, opening up like flowers as they do, while the marriage of the couple who introduced them falls apart. These are shy people, so Ryan is in polar opposition to her Gone Baby Gone role, but just as impressive.

Hmmmm … a fine actor, devoted to her craft, works with the top directors and playwrights on stage (Neil LaBute, Arthur Miller), plays Stella opposite John C Reilly in A Streetcar Named Desire, gets toothsome guest spots in Law & Order and ER, and then the movies come calling. That's all very well, but in my experience, often also a recipe for extreme worthiness and humourlessness (I invoke here the Edie Falco Principle). At least, I supposed so until Ryan turned up on the US version of The Office as Michael Scott's improbable true love, Holly Flax, and proved to have a molten core of absolute surreal goofiness burning within her. It's a strangely misshapen part – honestly, who could love Michael Scott? – that calls on Ryan, ditzily cute for a change, to bend herself into all kinds of shapes to maintain emotional plausibility – as only great actors can do. After her next movie, Win Win, the amateur actor playing her surrogate son was asked how a movie set struck him on the first day. He replied, "I was like, 'Dude, that's Holly from The Office!'" Fame at last.

If Amy Ryan was French or Scandinavian she'd be an Isabelle Huppert or a Liv Ullmann by now. If she was German they'd be throwing the Hanna Schygulla parts at her. Right now, in American films, no one is more interesting or as much fun to watch.